The viscosity is an important property of the gas, and it is greatly influenced by the temperature and pressure. Especially in a high-temperature and high-pressure environment, any small change in the temperature and pressure will largely influence the viscosity of the gas. Thus, it is crucial in scientific research and practical production to accurately acquire gas viscosity data under high-temperature and high-pressure condition. Methods for acquiring the gas viscosity include a calculation method and a measuring method.
The conventional gas viscosity calculation models include the Lennard-Jones model, the Stockmayer model, the Thodos model, etc., which are based on the theory of molecular kinematics under an assumption of rarefied gas. Thus, these models are only applicable to the gas viscosity calculation under the low-pressure condition rather than high-temperature and high-pressure condition.
The conventional viscosity measuring instruments include the capillary viscometer, the vibratory viscometer, the falling ball viscometer, and the rotational viscometer. Among these viscometers, the falling ball viscometer and the rotational viscometer are applicable to measure the liquid viscosity rather than the gas viscosity; the vibratory viscometer employs a quantitative relationship between the vibration attenuation and the measured fluid viscosity to measure the gas viscosity. Currently there is no gas viscosity measuring instrument which is practical under the high-temperature and high-pressure condition.
The capillary viscometer employs the Hagen-Poiseuille flow principle to measure the viscosity of the medium. At present, in the research reports on the capillary gas viscometers, the length of the capillary tube is usually increased to satisfy the measurability of the pressure difference which must be kept small enough to agree with the linear flow assumption of the H-P formula. In addition, due to the limitation of the micro-flow metering technology under the high-pressure condition, the difficulty in the micro-flow metering under the high-pressure condition is avoided by increasing the flow velocity. Under such test conditions of long tube and high flow velocity, it has to introduce an inlet (outlet) correction coefficient, a slippage correction coefficient, and a gas compressibility correction coefficient into the H-P formula. The uncertainty of the values of those correction coefficients brings many errors and an uncertainty to the measurement result.
In view of the above problems, based on the production and design experiences in this and related fields, the inventor has developed a high-temperature, high-pressure, and a low-velocity gas microtube viscosity measuring apparatus and a measuring method thereof, which can ignore the flow condition (flow velocity) and the device structure parameters (tube diameter and tube length) of the above correction coefficients, so as to solve the problems existing in the prior art.